1. Field of the Invention
The present invention relates to a magnetic recording medium, a production method thereof, and a magnetic recording device, and more particularly to a magnetic recording medium with a high coercive force and anisotropic magnetic field and a production method thereof, as well as a magnetic recording device provided with such a magnetic recording medium. The magnetic recording medium of the present invention can be ideally applied to hard disks, floppy disks, and magnetic tapes and the like.
2. Description of the Background Art
In recent years, magnetic recording media have been widely used as high density, large capacity recording media in devices such as hard disks, although improvements are now required in recording and playback characteristics in order to achieve even higher densities.
FIG. 10 and FIG. 11 are schematic illustrations showing a hard disk as an example of a magnetic recording medium.
FIG. 10 is a perspective view of a disc shaped magnetic recording medium, and FIG. 11 is a schematic cross-sectional view along the line A—A shown in FIG. 10.
A magnetic recording medium 90 shown in FIG. 10 comprises a disc shaped base material 91 of a non-magnetic material, and a metal underlayer 94, a ferromagnetic metal layer 95 and a protective layer 96 formed on top of this base material 91, as shown in FIG. 11.
In the magnetic recording medium 90 of this example, the base material 91 of a non-magnetic material utilizes, for example, a non-magnetic layer 93 comprising Ni—P provided on the surface of a substrate 92 comprising an aluminum based alloy or glass. A metal underlayer 94 of Cr for example, a ferromagnetic metal layer 95 comprising a magnetic film of CoCrTa or CoCrTaPt, and a protective layer 96 of carbon or the like are then layered sequentially on top of the base material 91. The typical thickness values for each of these layers are 5 μm to 15 μm for the non-magnetic (Ni—P) layer 93, 50 nm to 150 nm for the metal (Cr) underlayer 94, 30 nm to 100 nm for the ferromagnetic metal layer 95, and 20 nm to 50 nm for the protective layer 96. Although not shown in the drawings, a coating of a fluorine based lubricant such as perfluoro polyether may also be provided on top of the protective layer 96.
The inventor of the present invention have already reported [M. Takahashi, A. Kikuchi and S. Kawakita: IEEE Trans. on Magn., 33, 2938 (1997)] that in order to improve the recording and playback characteristics of a magnetic recording medium of the construction described above, a reduction in the interaction between the magnetic crystal grains which make up the magnetic film which functions as the ferromagnetic metal layer 95, and a reduction in the film thickness of the magnetic film, are essential.
In particular, in order to achieve a reduction in noise level for the medium, the above reference introduces a fabrication method effective for miniaturizing the magnetic crystal grains of the magnetic film, by reducing the film thickness of the ferromagnetic metal layer 95.
However, there are limits to the microstructure control and volume reduction in the magnetic grains which are possible by reduction in the thickness of the magnetic film comprising the ferromagnetic metal layer 95. The reason being that as the thickness of the magnetic film comprising the ferromagnetic metal layer 95 is reduced, there is an accompanying miniaturization of the crystal grains which make up the magnetic film, and a problem arises in that the magnetic characteristics, such as the magnetization (residual magnetization) recorded on the magnetic film, can vary significantly over time. In other words, the magnetic film becomes more susceptible to thermal agitation.
The inventor of the present invention have keenly pursued the development of a method for reducing the grain diameter of the crystal grains of a magnetic film by reducing the film thickness of the metal underlayer 94, in other words, a method of reducing the volume of the crystal grains of the magnetic film. Specifically, by developing a method of preparing a medium comprising a Cr underlayer and a Co based magnetic layer under ultra clean process conditions, the inventor succeeded in developing a medium capable of achieving a coercive force exceeding 2000 (Oe) even with an ultra thin Cr layer with a thickness of 2.5 nm as the metal underlayer 94 (International Patent Application No. PCT/JP97/01092).
However, it is thought that further miniaturization of the magnetization pattern recorded on the magnetic recording medium is required when the recording density is increased, and consequently, a magnetic film in which the magnetic characteristics such as the residual magnetization do not vary significantly over time, even if the grain diameter of the crystal grains which make up the magnetic film is reduced by reducing the film thickness of the magnetic film, in other words, a magnetic film which is capable of suppressing thermal agitation and ensuring thermally stable magnetic characteristics, has been keenly sought.